The present invention relates to an innovatory method and an apparatus for cooling and/or preserving perishable products under optimal conditions, and it refers in particular to fresh alimentary products or other materials different from the alimentary ones.
Low-temperature preservation methods are known in the art which consist in placing the products to be preserved into cooling containers, such as for example containers for goods transportation internally provided with evaporation panels of a refrigerating circuit for keeping low temperatures inside them. Due to the existence of discrete heat exchange surfaces, temperature in these containers is not at all uniform, as there are areas with a greater or lesser degree of cold depending on the distance from the evaporator and this also in the case in which air circulating systems are used within the container. In addition to local temperature variations it is also to be taken into account the fact that, due to their own nature, the above refrigerating systems have a non-eliminable hysteresis in controlling temperature inside the container, so that said temperature can oscillate within a rather wide range. The temperature constancy is also impaired by a virtually inexistent thermal storage offered by the cooling system. Short interruptions in the cooling system operation in fact give rise to rapid temperature increases in the container. In addition, the typical operation of these systems is of the on/off type, which results in continuous temperature oscillations.
Another undesired effect caused by the discrete heat exchange surfaces resides in that the heat exchanger has a remarkably lower temperature than the air temperature in the chamber, so that the humidity substracted from the products to be preserved condenses on the heat exchangers. For these reasons containers of the above type are well adapted to the trasportation of frozen goods, because for preserving them it is only important that a predetermined maximum temperature be not exceeded, the oscillations in the preservation temperature under this maximum value being on the contrary well tolerated and a reduction in the relative humidity in the container being quite irrelevant.
On the contrary, in order to ensure an optimal preservation for fresh products such as fruit, vegetables, cut flowers, seafood, meat, etc, they must be kept to a temperature as close as possible to the maximum freezing point, with deviations on the order of .ltoreq.1.degree. C. In order to achieve such results it is necessary to offer a very precise temperature regulation and a virtual elimination of the external sinusoid or at all events attenuation values better than 1:60. Any temperature variation different from such a minimum value therefore brings about worsening in preservation. In particular, temperature oscillations typical of conventional systems represent thermal cycles involving an accelerated aging of the products. In addition, any humidity subtractions from said products are very detrimental because they cause a quick withering and the forced ventilation systems of conventional containers (used for trying to keep the temperature gradients between the different points of the container sufficiently small) contribute to a rapid deterioration of the products, involving loss in weight and withering. This process is accelerated by the combined effect of the humidity subtractions due to the low (typically lower than 70%) relative humidity levels of the containers and a high (typically higher than 5 m/s) ventilation rate. In the Italian patent No. 1229358 filed on May 23, 1989 a refrigerated transportation means is disclosed which comprises a refrigeration circuit cooling an aqueous solution located on board of the transportation means and constituting a thermal accumulator. After the solution is completely frozen, the primary refrigeration circuit is disactivated and a secondary exchange device causes a brine fluid to circulate for a heat exchange between the thermal accumulator and exchange elements disposed within the container. By the above system an increase in the temperature steadiness on the exchange surfaces is achieved as well as the possibility of reducing the energy consumption over long periods of time, as the only necessary energy required is the small amount for operating the brine fluid circulation devices. However, the temperature steadiness by itself does not give satisfactory results in terms of best preservation of fresh products as the refrigerant system is at all events based on discrete exchange elements through which a brine fluid circulates.
U.S. Pat. No. 3,280,586 describes a portable cooler which has walls containing heat exchange elements spaced apart the same distance from each other. Each exchange element comprises a square box-shaped casing forming a cavity filled with thermal capacitance fluid into which an exchanger, in which a brine fluid circulates, is dipped. The brine fluid is circulated so that the heat exchange within the whole portable cooler takes place in a combined manner through the frozen thermal capacitance fluid and the thermal bridging existing between the brine fluid circuit and the wall. Thus the thermal accumulators sufficient to ensure a good stability in temperature on the exchange surfaces in contact with the portable cooler chamber are provided. U.S. Pat. No. 3,280,586 however does not take care of achieving a particularly low *.DELTA.T between the exchange surfaces and the air and, in addition, does not take care of having an as much as possible uniform temperature within the chamber. In fact, the exchange surfaces are still discrete surfaces and do not involve the whole of the portable cooler's inner surface. In addition, the different exchange elements have the brine circuit disposed in series and there are high temperature differences between the fluid inlet and outlet therein. As a result there is, among other things, the impossibility of embodying containers having relatively big sizes and wide exchange surfaces, because of the excessive pressure drops which would occur in the fluid circulation.
The foregoing, together with the important thermal bridges existing between the brine fluid and the inside of the portable cooler, which are not shielded from the thermal capacitance fluid in the cavities, creates localized areas of inacceptably low temperature. In addition, the brine fluid circuits dipped in the thermal capacitance fluid to be frozen have fins disposed in radial planes normal to the pipe axis, which prevents a uniform freezing of the thermal capacitance fluid from the brine fluid circuits to the wall not allowing a proper heat transfer between the thermal capacitance fluid and the portable cooler chamber. Thus there are areas in which ice bridges between the brine fluid circuits and exchange wall are formed, whereas other areas are still in a liquid phase. As a result, the areas on the inner walls of the container have different temperatures thereby giving rise to both temperature unevennesses in the chamber and formation of condensate, which will bring about subtraction of humidity from the inner environment.
As a matter of fact, the portable cooler described in the U.S. patent (at all events inadapted to undergo thermal expansions) is only useful if a limited thermal storage is to be supplied and is unable to control the temperature of the heat exchange walls. Therefore, it enables perishable goods to be quite well preserved only when it runs in a steady state, that is when the liquid in the cavities is completely frozen and the temperature of the goods is at the desired value within the chamber. On the contrary, it is completely inappropriate for cooling of the goods, that is when it is necessary to bring them to the preservation temperature starting from the external temperature for example, and to keep a constant temperature at all points in the chamber. Neither does it enable the partly melted liquid to be uniformly brought back to the solid phase so as to keep constant and uniform temperatures on the heat exchange surfaces with the portable cooler chamber. Therefore the system is useful as far as small portable coolers having reduced autonomy are concerned, for example those designed to operate over short distances for substantially local transportation and distribution of products, as recharging from the outside or installation of incorporated recharging systems is impossible (with the products inside).
Note should be also taken of that vegetable products have a high heat production (in the range of one hundred of watt per ton of products, for example). Therefore, known portable coolers that cannot be recharged in use and have restrained thermal capacitance and reduced air exchange surfaces can keep the inner temperature constant only over very short periods of time.
The general object of the present invention is to eliminate the above drawbacks by providing a method and apparatus for cooling fresh products and preserving them under optimal environmental conditions through the control of the wall temperature and consequently the inner air temperature.